The implementation of proposed IEEE 802.11 standards, and in particular the IEEE 802.11n standard, will allow for higher throughput (HT) wireless local area network (WLAN) devices. One such way in which higher throughput may be achieved is through the use of signal aggregation in both the medium access control (MAC) layer and the physical (PHY) layer. When an aggregate is addressed to a single receiver address, it is referred to as a Single Receiver Aggregate (SRA). When the aggregate is addressed to multiple receivers, it is referred to as a Multiple Receiver Aggregate (MRA).
An MRA may be transmitted during a Multiple Receiver Aggregate Multi-Poll (MMP) sequence or a Power Save Aggregation Descriptor (PSAD). This aggregation tends to improve system performance and also provides a power saving mechanism in the case of MMP/PSAD.
One or more MAC Service Data Units (MSDUs) being sent to the same receiver can be aggregated into a single Aggregate-MSDU (A-MSDU). This aggregation of more than one frame improves the efficiency of the MAC layer, particularly when there are many small MSDUs such as Transmission Control Protocol Acknowledgements (TCP ACKs). The overhead associated with channel access, such as the Physical Layer Convergence Protocol (PLCP) preamble, MAC header, and IFS spacing, can thereby be amortized over two or more MSDUs. Additionally, a STA may only use MSDU aggregation where it knows that the receiver supports MSDU aggregation. In some cases, support for MSDU aggregation may be mandatory at the receiver.
FIG. 1 shows an exemplary A-MSDU frame 10. The A-MSDU frame 10 includes a plurality of Sub-frame header fields 11 and a plurality of MSDU fields 12 (designated MSDU1 . . . MSDUn). Each Sub-frame header field 11 includes an MSDU length field 13, a source address (Source Addr) field 14, and a destination address (Dest Addr) field 15. Typically, the sub-frame header fields 11 separate the MSDU to aid a receiver in deciphering whether or not the frame is directed toward it. Ordinarily, the MSDU length field 13 includes the length, the Source Addr field 14 includes the address of the transmitter, and the Dest Addr field 15 includes the address of the receiver. In general, to form an A-MSDU 10, two or more MSDUs are aggregated together.
Another type of aggregation may be formed by joining multiple MAC Protocol Data Units (MPDUs) together. FIG. 2 shows an exemplary aggregated MPDU (A-MPDU) frame 20. The A-MPDU frame 20 includes a plurality of MPDU delimiter fields 21 and a plurality of MPDU fields 22 (designated MPDU1 . . . MPDUn). Each MPDU delimiter field 21 also includes a reserved field 31, an MSDU length field 24, a Cyclic Redundancy Check (CRC) field 25, and a Unique Pattern field 26. The A-MPDU frame 20 is typically transported in a single aggregate PLCP Service Data Unit (A-PSDU). Additionally, padding octets (not shown) are appended, if needed, to make each MPDU field 22 section a multiple of four octets in length, except in the case of MPDUn.
One purpose of the MPDU delimiter field 21 is to delimit the MPDUs 22 within the aggregate. For example, the structure of the aggregate can usually be recovered when one or more MPDU delimiters are received with errors. Also, individual MPDU delimiter fields 21 have the same block error rate (BER) as the surrounding MPDUs 22, and can therefore be lost during transmission.
One advantage in using A-MPDU frames 20 is that, unlike A-MSDUs, they can be aggregated to multiple receivers. That is, a multiple-receiver aggregate (MRA) may contain MPDUs that are addressed to multiple receivers. Moreover, an MRA may be transmitted in one of two contexts that are distinguished by whether it is transmitted during an MMP/PSAD sequence or not. If multiple responses are required, they may be scheduled by transmission of an MMP or PSAD frame.
FIG. 3 shows a typical multiple receiver aggregate multi-poll (MMP) frame 30. The MMP frame 30 includes a frame control field 31, a duration field 32, a receiver address (RA) field 33, a transmitter address (TA) field 34, a number of receivers (N) field 35, a receiver information (info) field 36, and a frame checksequence (FCS) field 37. The RA field 33 is typically the broadcast address of a group. The TA field 34 is typically the address of the wireless transmit/receive unit (WTRU) transmitting the MRA aggregate. The number of receivers (N) field 35 includes the number of receivers for which MPDUs are included in the MRA aggregate.
Additionally, the receiver info field 36 includes a plurality of subfields, such as an association identifier (AID) field 61, a transmission identifier (TID) field 62, a new PPDU flag field 63, a reserved field 64, a receive (Rx) offset field 65, an Rx duration field 66, a transmit (Tx) offset field 67, and a Tx duration field 68. The AID field 61 identifies a station (STA) addressed by the frame. The TID field 62 defines the TID for transmissions by a STA. The new PPDU flag field 63 indicates that the downlink (DL) for the STA begins at the start of the PPDU. The Rx offset field 65 defines the start of the first symbol containing DL data for a STA. The Rx duration field 66 defines the length of a downlink. The Tx offset field 67 defines the time when transmissions by the STA may begin, and the Tx duration field 68 defines the duration limit of the transmissions.
FIG. 4 shows a typical power save aggregation descriptor (PSAD) frame 40. The PSAD frame 40 includes a frame control field 41, a duration field 42, an RA field 43, a TA field 44, a basic service set identifier (BSSID) field 45, a PSAD parameter (PARAM) field 46, a number of receivers field 47, and an FCS field 48. The PSAD PARAM field 46 further includes a reserved field 71, a More PSAD indicator 72, and a descriptor end field 73. The number of receivers field 47 includes a plurality of individual Station Info fields which further includes a reserved field 81, a STA ID field 82, a downlink transmission (DLT) start offset field 83, a DLT duration field 84, an uplink transmission (ULT) start offset field 85, and a ULT duration field 86.
An MMP/PSAD frame may be transmitted as a non-aggregate, or may be aggregated with downlink MPDUs. Since the MMP/PSAD frame format defines receiving and transmitting durations for each STA, it enables STAs to save power since the STA can go into sleep mode when it is not either receiving or transmitting. Also, since the MMP sequence is protected using a network allocation vector (NAV) and extended PHY protection (EPP), MMP provides a mechanism of scheduling multiple transmission opportunities (TXOPs).
FIG. 5A shows an MMP/PSAD Downlink frame exchange sequence 50, and FIG. 5B shows an MMP/PSAD Uplink frame exchange sequence 55. In PSAD, a downlink transmission (DLT) and an uplink transmission (ULT) period of time are described by the PSAD frame 40. Which period of time is intended to be used for the transmission of frames from/to the PSAD transmitter to one of the PSAD receivers is also described in the PSAD frame 40.
In particular, FIGS. 5A and 5B show the start offsets for DLT1 to DLTn, and ULT1 to ULTn. Similarly, in MMP, offsets are shown for a series of downlink transmissions RX1 to RXn and uplink transmissions TX1 to TXn.
Aggregation is also possible at the PHY-level layer for physical layer (PHY) protocol data units (PPDUs). This aggregation may be referred to as an aggregated PPDU (A-PPDU). An A-PPDU contains one or more pairs of PLCP headers and PPDUs or PHY service data units PSDUs. To form an A-PPDU, two or more PPDUs (or PSDUs) are aggregated together, separated by the High Throughput Signal (HT-SIG) field.
FIG. 6 shows a typical aggregated PPDU (A-PPDU) 60. The A-PPDU 60 includes a legacy preamble (L-Preamble) 91, a High-Throughput Preamble (HT-Preamble) 92, a plurality of PSDU fields 93 (PSDU1 . . . PSDUn), and a plurality of HT-Signal (HT-SIG) fields 94 (HT-SIG1 . . . HT-SIGn). An HT-SIG field 94 may also include a length field 95, an MCS field 96, an advanced coding field 97, a sounding packet 98, a number HT-Legacy Training Field (HT-LTF) 99, a Short GI field 101, a 20/40 field 102, a cyclic redundancy check (CRC) field 103, and a tail field 104.
As shown in FIG. 6, the resulting A-PPDU 60 is therefore the combination of all PPDUs (or PSDUs) in the A-PPDU along with HT-SIGs 94 for each constituent PSDU 93. Since each PSDU 93 shown in FIG. 6 is delimited by an HT-SIG 94 that defines the various physical layers parameters, the A-PPDU comprises multi-rate PSDUs.
One of the drawbacks to the current system, however, is that when an MMP/PSAD is transmitted by the AP, it is possible that one or more of the STAs associated with the MMP/PSAD will not correctly receive, or incorrectly decode the MMP/PSAD frame. In these cases, the STAs that do not correctly receive or decode the MMP/PSAD frame will miss their scheduled uplink transmission times, effectively wasting the WLAN medium time.
It would therefore be advantageous if a method and apparatus existed that served as a mechanism to recover the structure of the A-PPDU 90 if one or more HT-SIGs 94 or PSDUs 93 are received in error due to poor channel conditions. It would further be advantageous for a method and apparatus to exist wherein an AP recovers any unused ULT, can transmit multiple MMP/PSAD frames, and can schedule multicast and broadcast transmissions in MMP/PSAD frames.